X-linked lymphoproliferative disease (XLP) is a devastating, inherited disorder in which the immune system responds abnormally and adversely to infectious agents, particularly Epstein-Barr virus (EBV). The disease is inherited in a strict X-linked recessive fashion, and afflicts boys at a very early age. XLP results in a compromised immune system, and is thus associated with high morbidity and mortality; patients often do not survive to their second decade of life. The underlying genetic defect of XLP is unknown, though the disease locus has been mapped to Xq25 between DXS982 and DXS75. Dr. Amemiya proposes to clone the XLP gene using the strategy of positional cloning. He has developed a novel, large-fragment cloning system that should facilitate the cloning effort. Candidate genes within the critical region of XLP will be tested on a panel of XLP patients to identify the XLP gene. Once cloned, the biological function of the XLP gene and the possible link between XLP and malignant lymphoma will be investigated. In addition, assays for clinical diagnosis and strategies for possible gene therapy will be developed.